It is known to convert methane into saturated and unsaturated, non-aromatic hydrocarbons having 2 or more carbon atoms, including ethylene, by means of a process called “Oxidative Coupling of Methane” (OCM). In this process, a gas stream comprising methane is contacted with an OCM catalyst and with an oxidant, such as oxygen or air. In such a process, two methane molecules are first coupled into one ethane molecule, which is then dehydrogenated into ethylene. Said ethane and ethylene may further react into saturated and unsaturated hydrocarbons having 3 or more carbon atoms, including propane, propylene, butane, butene, etc. Therefore, usually, the gas stream leaving an OCM process contains water, hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, propane, propylene, butane, butene and saturated and unsaturated hydrocarbons having 5 or more carbon atoms. In a case where in an OCM process air is used as oxidant rather than pure oxygen, nitrogen is also present in the gas stream leaving the OCM process.
In general, the conversion that can be achieved in an OCM process is relatively low. Besides, at a higher conversion, the selectivity decreases so that it is generally desired to keep the conversion low. As a result, a relatively large amount of unconverted methane leaves the OCM process. The proportion of unconverted methane in the OCM product gas stream may be as high as 70 to 80 mol % based on the total molar amount of the gas stream. This unconverted methane has to be recovered from the desired products, such as ethylene and other saturated and unsaturated hydrocarbons having 2 or more carbon atoms, which are also present in such gas streams. Further, as mentioned above, such gas streams may also comprise compounds like nitrogen, carbon monoxide and/or hydrogen, which have a boiling point which is lower than that of methane (“light compounds”).
It is known to separate the gas stream leaving an OCM process in the following way. Acid gas (mainly CO2) is removed in two stages, the first stage is an aqueous monoethanolamine (MEA) absorption system, and the second stage removes final traces of CO2 by scrubbing against aqueous NaOH. The CO2-free gas is dried in a dessicant bed and processed in a separation train similar to that used in conventional ethylene plants. The separation sequence comprises a front end demethanizer, deethanizer, C2 splitter, depropanizer, C3 splitter, and a debutanizer. The cryogenic needs for separation are met by a propylene-ethylene cascade refrigeration system that requires ethylene refrigerant only for the demethanization stage.
Thus, it is known to separate methane from saturated and unsaturated hydrocarbons having 2 or more carbon atoms, such as ethylene, by means of cryogenic distillation in so-called “demethanizer” columns. In cryogenic distillation, a relatively high pressure (in general: 23 to 35 bar) and a relatively low (cryogenic) temperature (in general: −120 to −70° C.) are applied to effect the separation of methane. The use of cryogenic distillation following an OCM process is for example disclosed in U.S. Pat. Nos. 5,113,032 and 5,025,108.
In a case where the gas stream also comprises light compounds such as nitrogen, carbon monoxide and/or hydrogen, it is known to first separate methane and the light compounds from the higher hydrocarbons by means of cryogenic distillation as discussed above. Then the light compounds are separated from the methane which is done by means of cryogenic distillation at a relatively high pressure (in general: 23 to 35 bar) and a relatively low (cryogenic) temperature (in general: −150 to −90° C.).
An object of the invention is to provide a technically advantageous, efficient and affordable process for recovering methane from a gas stream comprising methane and ethylene, more especially in a case where such gas stream comprises a relatively high proportion of unconverted methane, and wherein such gas stream also comprises light compounds such as nitrogen, carbon monoxide and/or hydrogen. Such technically advantageous process would preferably result in a lower energy demand and/or lower capital expenditure.